Remove the HasOwnBackingButPaintsIntoAncestor compositing state.

LayoutTests/compositing/squashing/no-squashing-into-fixed-position-that-clips-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [0, 50],
           "bounds": [800, 550],
           "contentsOpaque": true,
+          "drawsContent": true,
           "backgroundColor": "#ADD8E6",
           "children": [
             {
               "bounds": [800, 550],
               "children": [
                 {
                   "position": [400, -10],
                   "bounds": [24, 100],
                   "contentsOpaque": true,
                   "drawsContent": true,
                   "backgroundColor": "#D3D3D3"
                 }
               ]
             }
           ]
         },
         {
           "bounds": [800, 60],
           "contentsOpaque": true,
           "drawsContent": true,
           "backgroundColor": "#008000"
         }
       ]
     }
   ]
 }